The present invention generally relates to a multiple hearth furnace.
A multiple hearth furnace comprises an upright cylindrical furnace housing that is divided by a plurality of vertically spaced hearth floors in vertically aligned hearth chambers. A vertical shaft extends centrally through the hearth chambers, passing through each hearth floor. In each hearth chamber at least one rabble arm is fixed to the vertical shaft and extends radially outside therefrom over the hearth floor. Such a rabble arm is provided with rabble teeth, which extend down into material being processed on the hearth floor. As the vertical shaft rotates, the rabble arm moves over the material on the respective hearth floor, wherein the rabble teeth plough through the material and mix the latter. Depending on the angle of inclination of the rabble teeth, the material will be moved radially in towards the vertical shaft or outwardly therefrom. Drop holes are provided in each hearth floor, alternately in the inner zone of the hearth floor (i.e. near the vertical shaft) or in the outer zone of the hearth floor (i.e. near the cylindrical furnace housing). Material falling on the inner zone of a hearth floor is moved by the rabble arm radially outwards over this hearth floor, until it drops through a drop hole in the outer zone of this hearth floor on the outer zone of a hearth floor located directly below. On this lower hearth floor, material is moved by the rabble arm radially inwards until it drops through a drop hole in the inner zone of this hearth floor on the inner zone of the next lower hearth floor. Thus, material processed in the furnace is caused to move slowly along a serpentine path through the vertically aligned hearth chambers of the furnace.
It is a fact that multiple hearth furnaces possess major advantages over other solid material processing furnaces, such as rotary hearth furnaces, rotary kiln furnaces and shaft furnaces. By allowing a control of different hearth atmospheres and temperatures in the vertically aligned hearth chambers, they allow a very close control of the process inside the furnace. Other advantages of multiple hearth furnaces lie in their ability to maintain the processed materials in mixed condition throughout their passage through the furnace and to warrant a very intense exposure of the solid materials to process gases in a controlled gas/solid material counter flow within the furnace. Nevertheless, since their invention at the end of the nineteenth century, multiple hearth furnaces have only found very few applications in solid material processing. A reason for this lack of confidence in multiple hearth furnaces is that it has never been possible to warrant a problem-free operation of a multiple hearth furnace over longer periods.
One of the problems encountered with multiple hearth furnaces resides in the caking of material on the rabble arms, i.e. on the rabble teeth and their support structure. This caking of material on the rabble arms perturbs the operation of the hearth furnace, inter alia by bridge building between the rabble teeth, and is often a reason for severe damages to the rabble arms, the hearth floors, the vertical rotary shaft and the driving equipment of the latter.
A technical problem underlying the present invention is to provide a multiple hearth furnace in which excessive caking of material on the rabble arms can be efficiently avoided. This problem is solved by a rabble arm in accordance with the present invention.
A multiple hearth furnace in accordance with the present invention comprisesxe2x80x94just as prior art furnaces: an upright cylindrical furnace housing; a plurality of vertically spaced hearth floors dividing the upright cylindrical furnace housing into a plurality of vertically aligned hearth chambers; a vertical rotary shaft extending centrally through the hearth chambers; and at least one rabble arm associated with each of the hearth floors. These rabble arms are fixed to the vertical rotary shaft and arranged and designed so as to urge, during their rotation, processed material over the associated hearth floor towards a drop hole therein, wherein the processed material falls through the drop hole onto a lower hearth floor. In accordance with an important aspect of the present invention, the multiple hearth further comprises at least one cleaning lance inlet port and a cleaning lance assembly, which is associated with this cleaning lance inlet port. The cleaning lance inlet port is sealingly connected to the cylindrical furnace housing and leads radially into one of the hearth chambers. The cleaning lance assembly includes an elongated mount that is arranged outside the furnace housing and at least one cleaning lance slidably mounted on the elongated mount. The cleaning lance is connected to a cleaning fluid distribution system and comprises at least one cleaning nozzle. It can be sealingly introduced through the at least one lance inlet port along a radial trajectory into the hearth chamber by moving it along the elongated mount. The cleaning nozzle is arranged on the cleaning lance, so as to be capable of directing a jet of cleaning fluid onto a rabble arm that is positioned in a cleaning position in the vicinity of the radial trajectory of the cleaning lance when the latter is slidably moved along the elongate mount. It will be appreciated that the rabble arms can be very easily cleaned in this multiple hearth furnace, which of course helps to avoid the aforementioned drawbacks of prior art multiple hearth furnaces due to the caking of material on the rabble arms, i.e. on the rabble teeth and their support structure. In particular, cleaning of the rabble arms does no longer necessitate a cooling down of the multiple hearth furnace.
In a preferred embodiment, the cleaning lance assembly includes: a first and a second cleaning lance. The first cleaning lance is slidably mounted on the mount so that it can be introduced through a first lance inlet port in the cylindrical furnace housing along a first radial trajectory into the hearth chamber. The second cleaning lance is slidably mounted on the mount so that it can be introduced through a second lance inlet port in the cylindrical furnace housing along a second radial trajectory into the same hearth chamber. Both cleaning lances are connected to a cleaning fluid distribution system, wherein each cleaning lance comprises at least one cleaning nozzle. The first radial trajectory is determined so that the at least one cleaning nozzle of the first cleaning lance is capable of directing its jet of cleaning fluid onto the top of a rabble arm that is positioned in the cleaning position. The second radial trajectory is determined so that the at least one cleaning nozzle of the second cleaning lance is capable of simultaneously directing its jet of cleaning fluid laterally onto the rabble teeth of the same rabble arm in the same cleaning position, when the first cleaning lance and the second cleaning lance are moved along the mount. It will be appreciated that this embodiment allows to efficiently remove baked layers of material from the lateral and top surfaces of the rabble arms and from the rabble teeth.
The first cleaning lance and the second cleaning lance are advantageously mounted on a common lance support carriage that is slidably supported on the mount and driven by an endless chain mounted in the mount. In order to make the cleaning of the rabble arms even more efficient, the cleaning lance may comprise at its front end a lateral array of cleaning nozzles.
In order to enable a cleaning of the vertical rotary shaft from baked layers of material, the cleaning lance may comprise at its front end at least one radial cleaning nozzle that is capable of directing a jet of cleaning fluid radially onto the vertical rotary shaft.
The cleaning lance may further comprise an internal cooling circuit, which protects it against heat radiation in the hearth chamber.
In accordance with a first embodiment, the cleaning lance assembly is permanently supported in front of the same lance inlet port.
In accordance with a second embodiment, the cleaning lance assembly is supported by a vertical lifting device, so that it can be lifted to different hearth chamber levels. At each of these hearth chamber levels, the cleaning lance can then be introduced through a corresponding lance inlet port into the respective hearth chamber.
In an advantageous embodiment, the cleaning lance assembly is pivotably supported by the vertical lifting device, so that it can be rotated about a substantially vertical axis between an operating position, in which the at least one lance is substantially parallel to a central axis of a lance inlet port, and a lifting position, in which the at least one cleaning lance is substantially perpendicular to the central axis of the lance inlet port. It will be appreciated that the lifting position of the cleaning lance assembly allows to lift the latter from one hearth chamber level to the other, without interfering e.g. with a structural steel framework surrounding the hearth furnace and without endangering operational staff on platforms around the hearth furnace.
In a preferred embodiment, the vertical lifting device includes a vertical rail assembly that is supported so as to be capable of rotating about its vertical axis. A lifting carriage is slidably supported in this vertical rail assembly, and the cleaning lance assembly is supported by this lifting carriage. First drive means are provided for moving the lifting carriage along the vertical rail assembly; and second drive means are provided for rotating the vertical rail assembly by an angle of 90xc2x0 about its vertical axis.
In a preferred embodiment, the lance inlet port includes: a rigid inlet pipe connected to the cylindrical furnace housing and a ring-shaped lance sealing body connected to the rigid inlet pipe by means of a gastight flexible joint. The latter allows to compensate for misalignments of the cleaning lance and the lance inlet port.
The lance inlet port may further include a pivotable sealing flap, which seals it when the cleaning lance is withdrawn from the ring-shaped lance sealing body. This sealing flap further prevents material, which is urged by the rabble arms towards the outer wall of the furnace, from entering into the inlet port.
It remains to be said that the cleaning fluid is advantageously a gas-water mixture or mist, but that it is not excluded to chose a pressurised liquid, steam or gas as cleaning fluid.